a. Field of the Invention
This invention relates to methods and apparatuses for removing NOx, SO2, and Hg from a gas stream.
b. Description of the Related Art
Fossil fuels are burned in many industrial processes. Electric power producers, for example, burn large quantities of coal, oil, and natural gas. Sulfur dioxide (“SO2”), nitrogen oxide (“NO”), and nitrogen dioxide (“NO2”) are some of the unwanted byproducts of burning any type of fossil fuel. These byproducts are known to have serious negative health effects on people, animals, and plants, and a great deal of research has been done to find a way to economically remove them from flue gas streams before they enter the atmosphere.
SO2 is often removed from gas streams (“desulfurization”) by scrubbing the gas with an aqueous ammonium sulfate solution containing ammonia. Examples of this process are disclosed in U.S. Pat. Nos. 4,690,807, 5,362,458, 6,277,343, and 6,221,325, which are not admitted to be prior art by their mention in this Background section. The absorbed sulfur compounds react with ammonia to form ammonium sulfite and ammonium bisulfite, which are then oxidized to form ammonium sulfate and ammonium bisulfate. The ammonium bisulfate is further ammoniated to form ammonium sulfate. The process does not remove NO or NO2, however, which must then be dealt with using a different process.
NO and NO2 (together known as “NOx”) can be removed from a gas stream by contacting the gas stream with either ClO2 or O3 to oxidize NO into NO2, and then scrubbing with an aqueous solution of a sulfur-containing reducing compound of alkali metals or ammonia, and a catalytic compound. Such a process is disclosed in U.S. Pat. No. 4,029,739, by Senjo et al., which is not admitted to be prior art by its mention in this Background section. This process, however, does not remove SO2, and requires the addition of chlorine or ozone into the system by some other means.
Some processes exist that remove both NOx and SO2. In one such process disclosed in U.S. Pat. No. 4,035,470, by Senjo et al., which is not admitted to being prior art by its mention in this Background section, NO is oxidized to NO2 by contacting the gas with either ClO2 or O3 as above. Then the SO2 is scrubbed with a sulfite and an oxidation retardant that suppresses oxidation of the sulfite to sulfate. Iron or copper compounds can also be added to depress oxidation. Optionally, ammonium hydroxide can be added to make sulfite and to react with CO2 in the gas stream to make carbonate. Like in U.S. Pat. No. 4,029,739 mentioned above, this process requires the addition of either chlorine or ozone, and further requires a consumable sulfite oxidation retardant. However, both U.S. Pat. Nos. 4,029,739 and 4,035,470 require the addition of chlorine to a gas stream that is eventually released to the atmosphere, creating a serious safety concern.
Yet another process for removing NOx and SO2 from a gas stream is disclosed in U.S. Pat. No. 4,971,777, by Firnhaber et al., which is not admitted to be prior art by its inclusion in this Background section. In this process, NO is oxidized to NO2 by the addition of organic compounds which decompose into radicals at high temperatures. Then an aqueous ammonia solution in which the pH is adjusted to be below 5.0 absorbs the NOx and SO2. Scrubbing with ammonia produces a byproduct that may not be desirable in many industrial applications. Therefore, what is needed is a process that removes SO2, NO, NO2 and Hg from a gas stream that does not require the addition of a catalyst, chlorine, or ozone, and does not produce an ammoniated byproduct.